There has been known the following energy recovery/regeneration (recycle) device. To recover potential energy of a front working device for a working machine typified by, for example, a hydraulic excavator, the energy recovery/regeneration (recycle) device communicates between a bottom chamber and a rod chamber of a boom cylinder (hydraulic actuator) and regenerates pressure oil flown out from the bottom chamber of the boom cylinder to the rod chamber to boost bottom pressure of the boom cylinder while accumulating energy in an accumulator.
For example, the energy recovery/regeneration device described in Patent Literature 1 includes a pressure compensation valve for recovery and a recovery flow rate control valve on a route leading to an accumulator from a bottom chamber of a boom cylinder. The pressure compensation valve for recovery performs control so as to constantly maintain a differential pressure between before and after a meter-out throttle of the recovery flow rate control valve. This allows controlling a flow rate through the recovery flow rate control valve at a target flow rate according to an opening area of the recovery flow rate control valve without being affected by accumulator pressure, which is changed by the accumulation situation of the accumulator, thus controlling a contraction speed of the boom cylinder at a predetermined target speed.